Tube-era capacitor analyzers - Sprague Tel-Ohmike examples

Tube-era capacitor analyzers
Nearly all of the tube-era capacitor analyzers measure resistance and capacitance with a direct-reading bridge circuit. A current-limited AC voltage ranging from 6 volts to as high as 66 volts or more is used on the bridge. The eye tube indicator opens when the correct resistance or capacitance is selected by way of the main dial potentiometer. As with all bridge devices, accuracy is dependent on the precision of the standard capacitors and resistors in the device. While measurement with a modern digital meter is accurate and easy, typical digital meters cannot test for leakage at rated voltage. Therefore tube-era capacitance testers are still very much in demand. The Sprague Tel-Ohmike series are especially prized because of the built-in meter and the ability to measure insulation resistance and for later models, the push buttons that, when released, automatically zero out the voltage on the cap under test.

Sprague TO-16
Sprague introduced the first Tel-Ohmike, the TO-1 in early 1940. It did not come with a meter. The TO-16 was introduced later that year. Even though its model number is the highest in the TO series, the TO-16 is one of the first Tel-Ohmikes. It is basically the original TO-1 with an added meter section, all in one case. Like the basic TO-1, it uses a 5Y3 rectifier, a 6C5 triode, and a 6E5 eye tube.

Radio News September 1940 informational ad for Sprague TO-16

TO-16 repairs
I debated turning this TO-16 example into parts since the front panel had considerable surface rust. I decided to see what it would take to make it operational despite its cosmetic appearance. Scrubbing the panel with plastic scouring pads cleaned it up significantly. I was careful not to destroy the lettering. The plastic dial cover had warped somewhat. Removing it followed by a close encounter with a hair dryer got it back into shape.

As can be seen, the TO-16 has a built-in DC voltmeter/ milliameter that can be used for both internal measurement or external. Since the voltmeter is used to read the internal voltage for reforming a capacitor and determining leakage current, its accuracy is important. The series resistors for voltage measurement had drifted high, therefore the meter readings were low. I added some parallel resistors to bring the meter back to accuracy.

The TO-16 has a step switch as well as a potentiometer control for setting the capacitor leakage test voltage. The highest range for the step switch is 1000 volts and is not adjustable. All the other ranges in the step switch can be vernier adjusted by way of the potentiometer. The original electrolytics reformed well and the highest voltage did indeed measure 1000 volts. All of the step ranges were also accurate. The step switch resistors had drifted a bit but not enough to throw off the voltage for any of the steps.

Sprague TO-16 reforming an electrolytic

All of the Tel-Ohmike series and most other bridge capacitance meters use the same three values of capacitor for standards; those are 200 pF, 0.02 MFD, and 2 MFD. The 200 pF cap on the TO-16 and the other Tel-Ohmike units I repaired use a compression trimmer for that range. Those are seldom leaky. I had to replace the 0.02 cap (two 0.01 caps in parallel) but the 2 MF standard in the TO-16 which consists of two high-quality 1 MFD metal-sealed oil caps in parallel happened to be in good order.

I replaced the power cord with a three prong version, rewiring so that the line goes to the fuse and then to the power switch. The neutral is directly connected to the power transformer. All of the remaining wax paper caps were replaced. The 6E5 eye tube was in excellent condition. The meter was accurate although sluggish. I tried replacing the movement with one from a newer 1 mA meter but it did not fit well. I will look for a 1 mA meter movement of the same size and vintage. I used deoxit sparingly on the controls and switches.

Sprague TO-16 Gotcha! As designed, the voltage potentiometer on a TO-16 is live with high voltage on its shell including the knob control shaft. My TO-16 had one odd-ball knob on another control which was likely a replacement. That bakelite knob had a larger skirt and the set screw was deeper (farther from my fingers) than that of the other "chicken head" knobs on the set. It's the knob with the white stripe. I decided that its best use was for that potentiometer voltage control!

The TO-16 directions also note that after a voltage test, the step switch and the potentiometer should be returned to the zero voltage setting to discharge the cap under test. I suggest using an outboard digital voltmeter in parallel with the cap under test (AC for bridge measurement and DC for leakage) until you are totally familiar with the proper operation of the device.

After using the TO-16 a few times, I was quite impressed with its capabilities and decided to check out a couple of later models.

Sprague TO-4 Tel-Ohmike
The Sprague TO-4 is a 1952 version using surplus tubes; a 1629 eye tube, a 1619 as controlled rectifier, and a 12J5 triode. Like the TO-16, the meter resistors were off on the 600 volt scale. That scale uses a 200K resistor in parallel with a 150K resistor. The result is identified in the manual as R-8 with a value of 82K ohms. Both resistors are two watt carbon resistors and are easy to spot because of their size. One side of the resistor pair connects to the chassis. I again added some parallel resistance to bring the meter to an accurate reading. The 60 volt scale and the 6 and 60 mA current settings showed good accuracy. My TO-4 used rubber covered wiring to connect a couple of the selector switches to various chassis and transformer locations. The insulation had deteriorated. I replaced that wiring and used heat shrink tubing to gather the wires together as a bundle in a manner similar to the original.

Sprague TO-4 checking a 0.047 MFD capacitor

The 0.02 cap and 2 MF cap (a pair of large 1 MF caps in parallel) were replaced as well as several other caps. The power cord was replaced with a new 3 wire grounded version.

Sprague TO-4 testing electrolytic capacitor Pressing the left momentary contact button shifts the meter from its default leakage current range to voltage measurement

The TO-4 is fun to use. As clearly stated in the manual, for safety all push buttons must be out before touching a cap after testing. Again, I recommend hooking a digital meter in parallel with a cap to familiarize yourself with the AC voltage across the cap in bridge testing (when the eye tube lights on this model) and DC voltage when in leakage or insulation testing mode.

Sprague TO-5 Tel-Ohmike
The Sprague TO-5 was introduced in 1958. This example is 1961 vintage. The TO-5 is like the TO-4 but uses a printed circuit for its chassis wiring and substitutes a 6C4 triode for the 12J5 triode. Like the TO-4, I had to replace the 0.02 and the 2 MFD (again two large 1 MF in parallel) caps. Several other caps were also replaced. A three-wire cord was installed. The 600 volt meter scale multiplier resistors needed additional parallel resistance. While the TO-5 circuit is nearly identical to that of the TO-4, it adds a transformer ratio feature which is very useful. The ratio test uses the bridge function to determine the turns or impedance ratio of laminated transformers such as the typical audio, interstage or power transformer.

Answer: It's already over 1000 megohms on the way to over 1500 megohms insulation resistance. A very good cap.

The Semcor RC-115
Semcor of Hato Rey, Puerto Rico, made a nicely-designed slightly-smaller copy of the Sprague TO-5. Like the TO-4 and TO-5, I had to replace the 0.02 and the 2 MFD caps (again two parallel 1 MFD) used for standards as well as some other caps. A former owner had already replaced the power cord with a proper 3 wire version. The meter was accurate with its original resistors. Like the Sprague TO-5, it has that useful transformer turns/ impedance ratio function. Its only failing in my estimation is the lack of a pilot light. You cannot tell that the unit is powered unless the eye tube is showing green which only occurs for the bridge functions. The TO-4 and TO-5 both have pilot lights. With that clear plastic meter on the Semcor, a pilot light such as a bright LED could easily be mounted inside the case without front-panel modification.

Semcor RC-115 reforming a old Mallory 30 MFD 450V electrolytic capacitor sold by Lafayette Radio The 60 mA current meter range is the default. I keep the current well below 10 mA maximum.

Reforming voltage is now at the full 450V

Pressing the momentary contact 6 mA meter setting. The reforming current is already down to a bit over 2 mA on its way to less than 1 mA.

After reforming, the Semcor shows a power factor of about 1.5% for the electrolytic, still quite usable in a tube power supply circuit.

Using the Tel-Ohmikes The Sprague Tel-Ohmike capacitor analyzers (including the Semcor) are especially useful because of their built-in meters. They can be used to reform capacitors to a preset voltage while monitoring the actual leakage current. The power factor control gives a good sense of the quality of an electrolytic as used in tube circuits with typical low frequency filtering requirement (60 or 120 Hz ripple). The TO-5 and the Semcor clone have that useful transformer ratio function which unfortunately Sprague dropped from the later TO-6 and TO-6A.

I consider the Tel-Ohmike series to be the Cadillacs for tube-era cap testers. About the only digital tester made in recent years that is a better choice is my Sencore LC-75 Z-meter and its LC-10x relatives which are very expensive pieces. I was fortunate enough to acquire an LC-75 as a used item for a very reasonable price and used it to test and select the standard capacitor replacements for the Tel-Ohmikes.

Using a Sencore LC-75 (My fingers do NOT get this close when testing for leakage at higher voltages.)

Bob Putnak has a nice write-up on the TO-3. The TO-3 is similar to the TO-16 but does not use a step switch for high voltage. That places greater stress on the voltage pot. Note Bob's caution that the TO-3 voltage potentiometer also has the same "Gotcha" as the one on the TO-16, TO-2, and TO-1. Keep that well-insulated knob on the voltage pot! The TO-4 and later Tel-Ohmikes use a different circuit for voltage control that does not use a pot with live B+ on its frame.